As basic antennas, a dipole antenna, a monopole antenna, an inverted L antenna, and a slit antenna are known.
The dipole antenna is an antenna composed of two connected bars of conductor with quarter wavelength and is used as an antenna for FM broadcast or terrestrial television.
The monopole antenna is considered as a detached half of the dipole antenna and is used as an antenna for AM broadcast, transceivers, or mobile telephones.
The inverted L antenna is a modification of the monopole antenna and considered as an antenna having an antenna element bent at the base and widened. Since its shape may be small with respect to wavelength of radio wave and the antenna may be adapted to wide band, it is used as an antenna for cordless telephones or mobile telephones.
In the slit antenna, an elongated hole located on conductor constitutes the antenna and the length of the hole has an effect on the antenna characteristics.
As wide band antennas used in a UWB (ultrawideband) system, an antenna with a space structure typified by a double ridge guide horn antenna, a circular plate monopole antenna, or a planar dipole antenna in various shapes are disclosed in Non-patent Document 1.
On the other hand, since the UWB is a system covering wide frequency band in range of 3.1 to 10.6 [GHz], it is required to suppress interference with other systems with band in 5 [GHz] or the like.    Non-patent Document 1: N. P. Agrawall, et. al., IEEE Trans. Ant. Prop., Vol. 46, No. 2, 1998
In order to provide an antenna common to a plurality of frequencies, a plurality of antennas having each resonance frequency may be arranged, for example. However, this may bring a disadvantage that the antenna structure becomes relatively complicated. Accordingly, such disadvantage will be overcome when one antenna is common to a plurality of frequencies, but band covered by each resonance frequency is not generally a wide band.
In other words, there had been no antenna common to a plurality of frequencies, with simple structure, for covering a wide band assigned in the UWB system and suppressing interference with other systems.
For example, a monopole antenna with a simple structure having a slit is disclosed in Patent Documents 1 and 2; however, wide band characteristics and frequency sharing are not provided by those related arts.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-290139    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-37431
Further, in Patent Document 3, a wide band antenna device assuming the UWB system is disclosed. In this disclosure, a structure in which a plurality of element antenna patterns having different resonance frequencies is described; however, an antenna which is originally assigned for a narrow band is adapted to a wide band with use of multiple resonance.
That is, the antenna is adapted to a wide band by overlapping two or more narrow band resonances so that such antenna does not include a function for suppressing interference with other systems or filtering. Further, since feeding structure is required for each element antenna pattern, there has been a problem that the structure becomes complicated.    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-101342